US law enforcement agencies have launched a full-scale manhunt for a dismissed Los Angeles officer believed to have killed three people including another cop.

Christopher Jordan Dorner had posted a chilling online warning about "terminating" the life of a colleague he blamed for his dismissal, and threatening other police officers and their families.

The LA Police Department (LAPD), backed up by other agencies including the FBI, said it was increasing protection for more than 40 people named in a "rambling" online manifesto by Dorner, a 33-year-old US Navy reservist.

"Dorner is considered to be armed and extremely dangerous," LAPD chief Charlie Beck told a news conference on Thursday, US time, adding the suspect has "multiple weapons at his disposal, including assault rifles".

A Californian mountain resort ordered its schools to lock down after reports that a car resembling Dorner's was found burning in nearby woods.

Police were dispatched to the scene after the pick-up truck was found on Thursday near the town of Big Bear, in the mountains about two hours east of Los Angeles, the LA Times reported.

In other fast-moving developments, media reports said a military base near San Diego was also placed into lockdown, possibly as part of the manhunt. Beck declined to comment, but confirmed the threat was wider than just Los Angeles.

"This is a vendetta against all of southern California law enforcement, and it should be seen as such," he said.

Dorner was already wanted over the suspected revenge killing on Sunday of a couple, Keith Lawrence and Monica Quan, who was the daughter of Randy Quan, a retired police officer he blamed for his firing.

He then allegedly attacked two other officers overnight in Riverside, east of Los Angeles, killing one and injuring the other. Another officer was injured in a separate incident.

In addition, two civilians were injured early on Thursday when police officers opened fire on them in a vehicle they believed to be Dorner’s. "Tragically we believe that this was a case of mistaken identity," said Beck.

Dorner posted a multipage manifesto online on Monday, saying he was not afraid to die because he had already died when he was dismissed in September 2008 for making false statements about his training officer.

"I never had the opportunity to have a family of my own, [so] I am terminating yours," he wrote to Randy Quan.

"Suppressing the truth will [lead] to deadly consequences for you and your family. There will be an element of surprise where you work, live, eat and sleep," he said, referring to Quan and several others.

The LAPD said it took the threats "very seriously" and promised "all measures possible to ensure the safety of our LAPD personnel, their families and the Los Angeles community".

Dorner is described as black, 180 centimetres tall and about 120 kilograms. He is said to be driving a grey Nissan pick-up.

In his 20-page manifesto, published by ABC news and other media, Dorner said he was determined to clear his name.

"The attacks will stop when the department states the truth about my innocence, PUBLICLY I will not accept any type of currency/goods in exchange for the attacks to stop... I want my name back, period. There is no negotiation [sic]."

He also singled out lesbians and Asians as targets.

"Those lesbian officers in supervising positions who go to work, day in day out, with the sole intent of attempting to prove your misandrist authority (not feminism) to degrade male officers. You are a high value target.

"Those Asian officers who stand by and observe everything I previously mentioned... You are a high value target as well," he wrote.

Beck said Dorner’s police and military training made the threat all the more serious.

"He knows what he’s doing. We trained him ... he’s also a member of the armed forces," he said.

"It is extremely worrisome and scary."

Asked what message he wanted to give Dorner, he said: "I would tell him to turn himself in. This has gone far enough. Nobody else needs to die."

AFP

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